The invention applies to a method and an apparatus for the interferometric examination of scattering objects.
Scattering objects, in particular strongly scattering materials, can be investigated with optical coherence tomography (OCT), according to the current state of the art up to a penetration depth in beam direction of 3 mm. Together with a sampling, transversal to the beam direction, in one or in both dimensions, a 2D- or 3D-picture is generated.
During the interferometric sampling, a partial beam reflected by the object interferes with a partial beam reflected by a reference mirror which is movable in beam direction. Alternatively, the entire interferometer can be moved. The interference signals, converted opto-electronically by sensors, are demodulated to form a unipolar demodulation signal, used to obtain a digital image of the object. Envelope curves of the interference signals result. With respect to the current state of the art, reference is made to WO 97/27468 the contents of which is incorporated herein by reference.
The more broadband the irradiated light is, the smaller is the full width half maximum (FWHM) of the demodulation signal and thus the spatial resolution in depth direction. This, however, is impaired by dispersion due to wavelength-depending transit time differences of the received signals between object arm and reference arm. It is tried to achieve a dispersion compensation by balancing both optical arms. However, due to the unknown optical characteristics of the object to be investigated, which can even change depending of the penetration depth, this cannot be completely obtained. A complete compensation is hardly possible as both optical arms cannot be exactly the same. This becomes even more relevant if higher orders of dispersion have to be taken into account. Furthermore, additional elements used for compensation can cause negative results concerning the signal propagation in the interferometer.
In the field of medicine, optical coherence tomography can be applied for the examination of scattering objects accessible outside the body, as e.g. skin, nails, lips etc., or endoscopically, for the examination of the bronchial system, the gastrointestinal tract, or the lungs. In non-medicinal fields, the examination of thin, optically scattering plastic or ceramic layers is possible.
It has already been proposed to increase the spectral bandwidth by combining a plurality of central wavelengths, in order to increase the depth resolution. This is known for example from U.S. Pat. No. 5,795,295. Further interferometer arrangements working with two central wavelengths are known from U.S. Pat. No. 5,835,215, DE 19700592 A1 and WO 92/19930 A1. From DE 4429578 A1, use of intensity modulated light is known.
The invention addresses the problem of making available a method and a device for further increasing the spatial resolution in optical coherence tomography.
According to the invention, this problem is solved by a method for the interferometric examination of scattering objects, wherein intensity-modulated light is divided, one beam directed into an object and another beam directed to a reference mirror, the reflected light is lead to a detector module by which it is converted opto-electronically to an interference signal and this signal is evaluated, light of at least two different central wavelengths is irradiated into the object and onto the reference reflector and the converted interference signals of the central wavelengths are shifted relative to each other for compensating the expected dispersion of their phase position.
The intensity modulation of the irradiated light allows a considerable increase of the instantaneous maximum light intensity as compared to a continuous light beam, without an increase of the overall power and without an unfavorable increase of the device temperature and the object temperature. This also increases the detected interference signal, thus obtaining an improved signal-to-noise ratio. If a fast A/D-converter is used, e.g. 10 MHz, and for modulation frequencies of the interference signal in the 100 kHz range, an On/Off sampling ratio of 1/100 is obtained. The maximum light power can be increased in the reciprocal ratio (here: 100/1), without increasing the average overall power. This is particularly true for surface emitters, as e.g. surface emitting LED""s.
The intensity modulation of the signals of the at least two central wavelengths, is preferably phase-shifted relative to each other. The detected interference signals are preferably digitized and stored in a computer. Thereafter the saved digital values of both interference signals are shifted against each other in the computer, for dispersion compensation. The intensity modulation of the irradiated light, respectively of one wavelength, is preferably realized with a phase shift of about xcfx80/2 of the respective central wavelength. An exact offset of xcfx80/2 for different central wavelengths, however, cannot be obtained, and is not necessary.
According to preferred embodiments of the invention, a digital modulation of the light from the light source (by switching on and off), or a modulation with continuous intensity change may be used. In the latter case, the light is modulated, in particular, with a sinus wave. A disadvantage of this soft intensity modulation with sinosoidal control is a decrease of the effective sampling ratio, and thus, a decrease of the effective power gain. On the other hand, there are no high frequency components due to sharp pulse edges, as with digital modulation.
The invention generates an effective spectrum which offers much more bandwidth than the spectrum of the individual light sources. However, the dispersion is limited to the individual dispersion of the individual light sources (for one central wavelength). This results in the following advantages: Reduced dependence of the dispersion from the penetration depth of the light into the object; good dispersion compensation also for higher orders; no additional optical materials in one of the optical object arms or reference arms.
For a further FWHM reduction, a preferred embodiment of the invention proposes the formation of the magnitude difference between the added signals (in-phase-signal, Iin) and the subtracted signals (out-off-phase-signal Iout) from at least two interference signals with different central wavelengths, taking into account a weighting factor W, according to the following equation:
Iges=|Iinxe2x88x92W|Iout||xe2x80x83xe2x80x83(1)
The weighting factor should preferably be  less than 1 and  greater than 0.3. A corresponding difference formation unit may be provided in the computer of the apparatus of the invention.
A further embodiment comprises the formation of the signal magnitudes of at least two central wavelengths, and the incoherent addition of the magnitude signals.
As an interference signal can be divided into magnitude and phase, another advantageous variant of the method of the invention is characterized by that fact that the particle velocity (vp) in the object is determined relative to the (known) travel velocity vo of the reference mirror, deducted from the phase difference xcex94xcfx86=xcfx86(z1)xe2x88x92xcfx86(z2) according to
Vp/vo=(xcex/4xcfx80)xc2x7(xcex94xcfx86/xcex94Z)xe2x80x83xe2x80x83(2)
wherein xcex94Z=z1xe2x88x92z2.
The apparatus is equipped with phase and velocity determination units, for determining velocities vp in the object, according to this method.